This invention relates to reference electrodes for use in high temperature environments which may involve high thermal shock conditions and more particularly to reference electrodes with protective structures over the electrode membranes. Specifically, the invention relates to reference electrodes useful in mixed halide salts as high temperature electrolytes in electrolytic cells for the electrorefining of spent reactor fuels.
In the electrorefining of two or more metals involving the selective deposition of at least one of the metals, a reference electrode is useful as a standard potential in the study of the process and to control the cathode potential without dependence on changing anode potentials. Control of the cathode potential is important in determining which metal of the metal mixture is being deposited on the cathode at a particular stage of the process. One new reactor system under development and evaluation by the U.S. Department of Energy involves the use of a metal fuel of U-Pu-Zr. Processing of spent fuel is carried out electrolytically during which U and Pu may be separately deposited on the cathode. The cell electrolyte is a mixed halide salt and the cell operating temperature is in the order of about 500.degree. C. The anode is a pool of liquid cadmium with the spent fuel below the electrolyte in the cell. The cell is operated in an inert atmosphere of argon with less than 2 ppm of water vapor and less than 2 ppm of oxygen. An earlier version of the cell is disclosed in U.S. Pat. No. 4,596,647 which is hereby incorporated herein by reference.
The reference electrode previously utilized and tested in the experimental electrolytic cell has been an alumina tube with a small hole drilled in the bottom for ionic access between the cell electrolyte and the electrolyte of the reference electrode. The reference electrode was based on a Ag/AgCl electrode with added electrolyte usually having the same composition as the nonreactive components of the main salt electrolyte of the electrolytic cell. The silver electrode was a wire with a lower end formed into a small coil or helix and was primarily retained in the tube except for an exposed upper end section for electrical connection. In general, this reference electrode had a large drift rate and required frequent regeneration by an anodizing step. These problems appeared to be due to a diffusive transport of silver chloride through the small opening (although packed with yttria fiber) at the lower end of the reference electrode.
Accordingly, one object of the invention is an electrolytic cell with a reference electrode operable for an extendable period of time at temperatures in the order of 500.degree. C. A second object of the invention is an electrolytic cell with a reference electrode operable in the cell containing a molten chloride salt electrolyte and a liquid metal anode pool containing uranium, plutonium, zirconium, sodium and other metals. Another object of the invention is a reference electrode with a glass electrode which is usable in a high thermal shock environment. These and other objects will be readily apparent from the following description.